The present invention relates to a detection circuit which is configured for detecting an electrical capacitance of an electrode device in an electrostatic holding device with a clamp carrier, in particular for detecting a component held by the electrostatic holding device. The invention furthermore concerns an electrostatic holding device provided with the detection circuit, and a method for detecting the electrical capacitance of an electrode device in an electrostatic holding device using the detection circuit. Applications of the invention lie in the operation of electrostatic holding devices.
Holding devices for electrostatic holding of components, also known as electrostatic holding devices, electrostatic clamping devices, electrostatic clamps, electrostatic chucks or ESC's, are generally known. One important application of electrostatic holding apparatus is in holding semiconductor wafers, in particular silicon wafers, in lithographic semiconductor processing, for example in chip production. Particular advantages are the easy switchability of electrostatic holding forces, a high positioning accuracy and a reliable fixing of the components in vacuum.
An electrostatic holding apparatus has a clamp carrier with at least one free surface for receiving the component, and an electrode device integrated in the clamp carrier with at least one pair of clamp electrodes (holding electrodes). The electrostatic holding forces are generated when the clamp electrodes are subjected to a high voltage and the clamp carrier is correspondingly electrically charged. The charged clamp carrier attracts the component so that this adheres to the free surface of the clamp carrier (so-called “clamped state”) and can be held and/or moved with this. To release the component, the clamp carrier is discharged and optionally recharged, i.e. charged with reversed polarity (so-called “declamping”).
By means of capacitance measurement at the clamp electrodes, or additional measurement electrodes of the electrode device, it can be detected whether, and if so where, a component is located on the clamp carrier, and whether the component is in the clamped state. For example, US 2002/0008954 A1 proposes coupling the electrical capacitance provided by the clamp electrodes at high voltage potential, via blocking condensers, to an oscillator at low voltage potential. If the capacitance of the clamp electrodes changes when the component is placed in position, the oscillator is measurably detuned. This technique however disadvantageously requires a capacitance of the blocking condensers which is 10 times higher than the capacitance of the clamp electrodes, whereby detection of small capacitance changes is hindered.
According to U.S. Pat. No. 6,075,375 A, measurement electrodes, which additionally are mounted on the clamp carrier next to the clamp electrodes, are connected to a capacitance detection circuit which contains a reference oscillator and a phase control circuit (PLL circuit) with a comparator and a voltage-controlled oscillator (VCO). The comparator compares the frequencies of the reference oscillator and the VCO, wherein the comparator output signal is fed back to the VCO as a control voltage. The capacitance presented by the measurement electrodes is connected to the reference oscillator which is detuned when the component is placed on the clamp carrier. The modified frequency influences the control voltage at the VCO which is analyzed as a measure of the capacitance to be detected.
The technique according to U.S. Pat. No. 6,075,375 A has the disadvantage that each PLL circuit has a specific capture range (so-called locked state of the VCO) which is determined by the capacitance to be detected. In practice however, different capacitance ranges are detected in different operating phases of the holding device. For example, when a wafer is placed on the clamp carrier, capacitance changes in the range of a few nF occur, while transition to the clamped state leads to capacitance changes in the range of only 1/10 nF. Furthermore, the value of the capacitance to be detected is dependent on the material and size of the component to be detected. As a result, the conventional detection circuit may leave the locked state of the VCO and give a false result. To avoid this problem, in the conventional technology it is necessary to adapt the circuitry of the capacitance detection circuit to suit the task in hand and the respective application conditions.
The technique used in U.S. Pat. No. 6,075,375 A furthermore has the disadvantage that the capacitance is measured at the measurement electrodes. These require a special surface on the clamp carrier which restricts the function of the clamp carrier. Furthermore, no information on the state of the component is obtained directly at the clamp electrodes.
Further systems are known in practice in which the capacitance to be detected is loaded with an alternating voltage, and the signal current resulting from phase-selective rectification at the capacitance is analyzed. These techniques however are imprecise, in particular for materials with low conductivity.
The objective of the invention is to provide an improved detection circuit and/or an improved method for detecting an electrical capacitance, in particular a capacitance change, of an electrode device in an electrostatic holding device with a clamp carrier, in particular for detecting a component held by the electrostatic holding device, in which disadvantages of conventional detection circuits are avoided and which is distinguished in particular by a wider area of application, a lower dependence on the value of the capacitance to be detected, a simplified adaptation to different application conditions, and/or an increased measurement precision. The objective of the invention is furthermore to provide an improved electrostatic holding device with which disadvantages of conventional holding devices are avoided and which is distinguished in particular by increased reliability in the detection of a component in various operating phases of the holding device.
These objectives are achieved by a detection circuit, an electrostatic holding device and/or a method of the invention.